Question on WordPress User Sync

I am building a website network that has three distinct components, but all feeds from one main landing page. is the main landing page that we are going to advertise. From there, the user chooses one of the other three sites to visit.
The 'coupon' site is strictly designed for Groupon-style coupons codes.
The 'stuff' site is regular retail sales.
The 'overstock' site is highly discounted sales.

All three are individual domains and individual WP installations.

I would like to simplify the process for someone if they are coming to our sites by allowing them to register on one of the three sites and have that registration content create the same userID and password information on the other sister sites.

I looked at WordPress User Sync and it appears to allow that functionality. I just wanted to get some clarification from someone who has used the plugin to confirm that is what the plugin does.

Also, does one site have to be the 'master' for all, or can each site be a master for the others. Meaning -- do they have to register on Coupons in order for the info to feed to Stuff and Overstock, or could they register on Stuff and have that create the accounts on Coupons and Overstock, too.

Thanks in advance for your help.